The mistress of Gambino boss Peter Gotti was found dead yesterday less than a week after she vaulted to fame as the mobster’s moll, police said.

Marjorie Alexander was found fully clothed with a plastic bag over her head – duct-taped around her neck – at a Red Roof Inn in Westbury, L.I., yesterday afternoon.

Alexander, a big-haired blonde, was a fixture at Gotti’s racketeering and money laundering trial last year and wrote six letters to Judge Frederic Block, begging for leniency for her beau.

Cops said it appears she took her own life.

“There was no apparent violence. There is no indication of foul play,” said Nassau County Homicide Detective Sgt. Richard Laursen. “We’ve recovered several notes at several locations.”

Alexander, 43, had been on Prozac since September 2003, according to one of the sentencing letters she wrote on behalf of her imprisoned paramour.

“I don’t have much spirit left, despite my being medicated with Prozac since September,” she wrote in January 2004. “I don’t think they had such circumstances in mind when they developed this lifesaving medication.”

Laursen said the cause of death was “undetermined,” pending an autopsy.

“Except for the fact she was Gotti’s girlfriend, this was a typical suicide situation,” Laursen said.

At least two suicide notes were sent by mail, Laursen said, calling them “typical suicide notes.”

Gotti’s lawyer told The Post early this morning his client had yet to be informed of the death.

“He’s going to be devastated,” lawyer Gerald Shargel said.

Hotel staff found Alexander in Room 301 at around 12:30 p.m.

Sources say she also left a note – and tip – for the hotel housekeeper.

Alexander, who lived in Rockville Centre with her estranged husband and their two teen children, checked into the nondescript $99 hotel room under her own name Monday night and asked not to be disturbed, telling hotel staffers she didn’t want any incoming phone calls.

Her family reported her missing on Tuesday.

In one of the letters to the judge, she identified herself as “the very significant other of Peter Gotti,” and as Gotti’s fiancée in another, although the late Dapper Don’s brother is still legally wed to Catherine Gotti.

Gotti, 64, who faces up to 15 years in prison when he is sentenced, is in the middle of a divorce from his wife after more than 40 years of marriage.

Catherine Gotti submitted just one letter to the judge – begging Block to nail her husband with a heavy sentence.

But Alexander, in a loopy, girlish script littered with superfluous punctuation, last year painted a much rosier relationship with Gotti, replete with plans that were derailed after he was convicted.

“It was our intention upon his just acquittal to leave the state. To live out our remaining years together in peace and tranquility,” Alexander wrote in March 2003. “Our children and grandchildren would come visit us.”

Gotti’s legal travails have left him depressed and his lawyers have asked the judge to give him a lighter sentence because of his condition.

“I have been the sole source of sanity for this man, and I’m on Prozac!” wrote Alexander, who spoke to Gotti every day and visited him in jail once each week, despite her aversion to the Metropolitan Detention Center in Brooklyn, where Gotti is being held in the mental ward.

“I still shake like a leaf every time I enter the MDC. I guess Jewish girls and jail aren’t a very good mix.”

Peter Gotti, a former sanitation man, became acting boss of the Gambino crime family in 1999, while his famous brother, John, was serving a life sentence.

John Gotti’s son, John Jr., preceded Peter as acting boss, but his rein was cut short by a 1998 federal racketeering indictment.

Peter Gotti became the bona fide boss when his brother died in 2002, but in one of the letters, the ever-loyal Alexander swore on her mother’s soul he was not the Gambino godfather.

In January, she thanked Block for “reading yet another letter from this most weary writer” and in a brutally frank sentence, summed up what had become of her life.

“My good character and spotless record mean nothing, anymore, for I am guilty of loving an inmate.”